CLINICAL PHARMACOKINETICS -2

Dr. Jayesh Vaghela

104-Jan-14

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Coping with Exposure to Xenobiotics

• Humans come into Contact with foreign chemicals, medicines, or Xenobiotics (substances foreign to body) Through –

- Intentional exposure,

- Accidental exposure,

- Diet.

• Fortunately, a mechanism to rapidly eliminate them is developed

⇓• They do not accumulate in & harm the body

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BIOTRANSFORMATION

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Topics to be discussed

Introduction to Biotransformation

The Phases of Drug Metabolism

Enzyme Induction & Inhibition

Enzymes Metabolising Xenobiotics

Sites of Drug Metabolism

Factors affecting Drug metabolism ( Biotransformation )

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BIOTRANSFORMATION

• Definition :-

“ Biochemical Transformation of

Drugs within the Living Organism,

catalyzed by Enzymes. ”

• Aim :-

Water insoluble → water soluble → easily excreted

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• Advantage :

Critical for survival

• Disadvantage :

Inter-individual variations

Drug-Drug interactions

Toxic & Carcinogenic derivatives

Species differences → limits the use of animal models in new drug

development

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Consequences of Biotransformation

Active Drug Inactive Metabolite :⇒Phenobarbitone → Hydroxyphenobarbitone

( Active Drug ) ( Inactive Metabolite )

Inactive Drug ( Prodrug ) Active Metabolite :⇒L-Dopa → Dopamine

( Prodrug ) ( Active )

Active Drug Equally Active Metabolite⇒Diazepam → Oxazepam

( Active ) ( Active )

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First-Pass Metabolism

• Drug Oral administration G.I.T. Portal circulation ⇒ ⇒ ⇒ ⇒ Liver ( First pass metabolism ) Systemic Circulation⇒• Decreases Bioavailability

• Decreases Therapeutic Response

• Can be bypassed if drug is given –

- Parenterally ( i.v. Xylocaine in Arrhythmias )

Or

- Sublingually ( Isosorbide dinitrate in Angina )

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Drugs Undergoing First-pass Metabolism

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Liver IntestineBronchial Mucosa

• Morphine• Xylocaine• Imipramine• Propranolol• GTN

• L- Dopa• Testosterone• Progesterone• Chlorpromazine

• Nicotine• Isoprenaline

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Site Of Biotransformation

In Cell

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Chemical Pathways Of

Drug Metabolism

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THE DRUG METABOLIZING ENZYMES

(1) Microsomal Enzymes :

• Location : - Smooth Endoplasmic Reticulum of Liver cells,

- Intestinal mucosa, Lungs, Kidney.

• Activity : - Phase 1 reactions

- Phase 2 reaction ( Glucuronyl conjugations )

• Example : - Cytochrome P-450 enzymes

• Metabolize only Lipid-soluble drugs

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(2) Non-Microsomal Enzymes :

• Location : - Cytoplasm,

- Mitochondria of Hepatic cells,

- Plasma.

• Activity : - All Phase 2 reactions ( Except Glucuronyl conjugation )

• Example : - Esterase, - Monoamine Oxidase (MAO),

- Amidase, - Transferase.

(3) Non-enzymatic ( Hofmann ) Elimination :

• Spontaneous Molecular rearrangement, without any enzyme action

• Example - Atracurium

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Phases of Drug Metabolism

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Phase 1 Phase 2

• Introduction of functional groups. -OH, -COOH, -SH ,-NH2

• Alters biological property

• Oxidation/Reduction Reactions

• Enzymes - CYPs, FMO

• Slower reaction rate

• Conjugation of substrate

• Alters Solubility and mol. wt.

• Conjugation / Hydrolysis

• Enzymes - Transferases, EH

• Faster reaction rate

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PHASE-1 REACTIONENZYMES

CYP ( Cytochrome-P450 ) FMO ( Flavin-containing Monooxigenases ) Hydrolytic enzymes

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Cytochrome P-450• Location :- Endoplasmic Reticulum

• Structure :-

Heme Polypeptide chain⇿• Nomenclature :

- CYP → Digit for Family → Letter for Subfamily → Gene number

- e.g. – CYP3A4 ⇒ - Cytochrome P-450

- Family “ 3 ”

- Subfamily “ A ”

- Gene Number “ 4 ”04-Jan-14

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Functions

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Xenobiotic Metabolism Endogenous Steroid Synthesis

• Capacity to metabolize diverse chemicals

⇓

• Sacrifices metabolic turn-over⇓

• T1/2 of drugs 3 – 30 hrs∼ Endogenous compounds = Sec/Min Dopamine, Insulin.

• Substrate specificity

• CYP 19 / Aromatase

⇓• Testosterone → Estrogen

⇓• Can not metabolize xenobiotics

CYP Family Main Functions

CYP1 • Xenobiotic metabolism

CYP2• Xenobiotic metabolism,• Arachidonic acid metabolism

CYP3 • Xenobiotic and steroid metabolism

CYP7 • Cholesterol 7 -hydroxylationα

CYP11• Cholesterol side-chain cleavage,• Steroid 11 –hydroxylation,β• Aldosterone synthesis

CYP17 • Steroid 17 -hydroxylationαCYP19 • Androgen aromatization

CYP21 • Steroid 21-hydroxylation

CYP24 • Steroid 24-hydroxylation

CYP27 • Steroid 27-hydroxylation04-Jan-14 18

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CYP Enzymes in Metabolism of SomeClinically Important Drugs

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CYP Inducer InhibitorDrugs

metabolized

• 3A4• 3A5

• Barbiturate• Rifampicin

• Erythromycin• Ketoconazole

• Acetaminophen,• Caffeine

• 2D6 -- • Quinidine • Codeine

• 2C8• 2C9

• Barbiturate• Rifampicin

• Fluconazole• Phenytoin• Warfarin

• 2C19• Barbiturate• Rifampicin

--• Diazepam• Omeprazole

• 1A1• 1A2

• Barbiturate• Rifampicin

--• Theophylline• Warfarin

• 2E1 • Alcohol • Disulfiram• General

anaesthetics

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PHASE-1 REACTIONS Oxidation Reduction Hydrolysis

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CYP-dependent Oxydation Reactions(1) Aromatic Hydroxylation:

Phenytoin

Warfarin

Ethinyl estradiol

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(2) Alliphatic Hydroxylation :

Ibuprofen,

Phenobarbital,

Cyclosporin,

Midazolam

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(3) Dealkylation :

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N- dealkylation :

• Morphine• Theophylline

O- dealkylation :

• Codeine

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• Deamination

• Amphetamine • Diazepam

Carbinolamine intermediate

• Dehalogenation

• Halothane

Hepatotoxicity

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•N-Oxidation

• Dapsone

• Clozapine

• Mianserine

•S-Oxidation

• Chlorpromazine

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Cytochrome P-450 Independent reactions

FMO ( Flavin-containing Monooxygenase )

EH ( Epoxide Hydrolase )

Hydrolysis

Reduction

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FMO ( Flavin-containing Monooxygenase )

• Location :- Endoplasmic Reticulum

• 6 families → FMO3 is most important

• Drugs metabolized : - Nicotine, Cimetidine.

• Deficiency causes “ Fish odor syndrome ”

Trimethylamine N-oxide ( TMAO ) → Trimethylamine

• Not induced or inhibited by drug

⇓• No drug-drug interactions

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( EH ) Epoxide Hydrolase

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Carbamazepine

Carbamazepine EPOXIDE

Carbamazepine Di-hydrodiol

Valproate inhibits

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Reduction

• Digoxin

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Antibiotics

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PHASE -2 REACTIONS Glucuronidation

Sulfation Glutathione conjugation

N-acetylation Methylation

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Glucuronidation

• Enzyme : UGT ( UDP-glucuronosyltransferases )

• Co-factor- UDP-glucuronic acid

• Location : - Liver,

- G.I. epithelial cells

• UGT-1 is more important than UGT-2 in drug metabolism

• drugs metabolized : - Morphine,

- Diazepam,

- Digoxin.

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Gilbert’s Syndrome

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Irinotecan

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Sulfation

• Enzyme : - Sulfotransferase

• Co-factor : - Phosphoadenosyl phosphosulfate

• Drugs : - Methyl dopa

- Ethinyl estradiol

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Glutathione Conjugation

• Co-factor : - Glutathione(GSH)

• Enzyme : - GST

(Glutathione-S-Transferase)

• Normally, High ratio of

GSH : GSSG is needed

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N-Acetylation

• Co-factor : - Acetyl CoA

• Enzyme : - N-Acetyl Transferase ( NAT )

• Drugs : - Amide-containing drugs,

e.g. – Isoniazide,

- Sulfonamide,

- Dapsone,

- Clonazepam,

- Benzocaine.

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Methylation

• Cofactor : - S-Adenosylmethionine [ SAM ]

• Enzyme : - Methyltransferase ( MT ) (cytosol)

• Highly substrate specific

• Drugs :

- TPMT--- Azathioprine, 6-MT, thioguanine

- COMT --- dopamine, methyl dopa, nor- epinephrine

- HNMT --- Histamine

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INDUCTION OFDRUG METABOLISM

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PXR – Pregnane X receptor,RXR – Retinoid X ReceptorLigand – Atorvastatin

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Clinical relevance(A) Decreased Plasma Concentration & Therapeutic effect :

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Enzyme inducer Induced enzyme Drugs affected• Phenobarbitone• Phenytoin• Carbamazepine

• CYP3A4• Barbiturates• Vit D• Theophylline

• Rifampicin• Phenobarbitone

• CYP3A4• CYP2C9

• Oral contraceptives• Warfarin

• Smoking• Omeprazole

• CYP1A2• Theophylline• Warfarin

• Ethanol (chronic)• Isoniazide

• CYP2E1• Ethanol itself• General anesthetics

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(B) Drug Toxicity :

Ethanol drinkers

⇓Paracetamol

⇓N-acetyl-P-benzoquinoneimine

⇓Hepatotoxicity

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(C) Therapeutic benefits :

Phenobarbitone given 7 – 14 days before labour

⇓Induces foetal hepatic glucuronyl transferase enzyme

⇓Conjugation of bilirubin to glucuronic acid

⇓Excreted through bile

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ENZYME INHIBITION

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Clinical consequences

(A) Increased plasma concentrations & toxicity :

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Inhibitor Enzyme inhibited Drugs affected

• Cimetidine• Hepatic microsomal

MFO(Mixed Function Oxidase)

• Phenytoin• Warfarin• Theophylline

• Allopurinol • Xanthine oxidase • Azathioprine

• MAO inhibitors • Monoamine oxidase• Morphine• Tricyclic

antidepressants

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(B) Therapeutic benefits :

L- dopa + carbidopa

Disulfiram + Alcohol Alcohol aversion therapy⇒

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Factors affecting drug metabolism Age :

Neonates :

• Low activity of glucuronyl transferase enzyme

⇓

Choramphenicol

⇓

“ Gary baby syndrome ”

Elderly :

Decreased hepatic blood flow

⇓ Propranolol, Pethidine toxicity

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Sex :

Nutrition & diet

Disease :

Genetic variation :

Drug – rug interactions :

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Role of Drug Metabolism in theDrug Development Process

• Before filing the NDA for a new drug,

⇓Enzymes involved in metabolism

Potentiality of metabolites

• long-term carcinogenicity studies in rodents for drugs to be used chronic diseases.

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References

• HL Sharma & KK Sharma, Principles of Pharmacology, 2nd Edition,

Page 37 – 45

• Goodman & Gilman’s The Pharmacological Basis of Therapeutics, 12th

edition, Page 121 – 143

• KD Tripathi, Essentials of Medical Pharmacology, 6th Edition

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